Article Figures & Data
Figures
- Figure 1.
CD33 immunohistochemistry in human brain. CD33-immunopositive cell profiles (arrows) show morphology consistent with microglia in both AD and non-AD samples (a, b). Immunofluorescence was used to help distinguish the CD33-immunopositive cell types. c/d, e/f, and g/h show the same microscope fields. Sections were immunostained for CD33 and counterstained for IBA-1 (a microglial/macrophage lineage marker) or GFAP (an astrocyte lineage marker), as indicated. Microglia with rounded morphologies tended to colocalize with both CD33 and IBA-1 (blue arrows in a–d), with more ramified IBA-1-immunopositive microglia staining less positively for CD33 (yellow arrows); these results suggest that CD33 expression is increased in ameboid microglia. In addition to apparent double labeling in microglial cells, there are areas in which GFAP-positive label colocalizes with CD33 immunopositivity, indicating either astrocytic expression of CD33 or the engulfment of CD33-positive cells by astrocytes (note the two DAPI-labeled nuclei present in the CD33-positive/GFAP-positive area indicated by arrows in g, h). Sections are from superior/middle temporal gyri of individuals with AD pathology (a, c–h) or without (b) AD pathology. Ctrl, Control. Scale bar: a, b, 50 μm; c–h, 20 μm.
- Figure 2.
CD33 isoform expression relative to AD status. CD33 expression correlated well with microglial gene expression (a, presented as geometric mean of CD11b and AIF-1, r2=0.64), as well as each microglial mRNA individually (data not shown). An association between CD33 and AD status was visualized by considering the ratio of CD33 to the geometric mean of the microglial reference genes (b). D2-CD33 correlated well with CD33 expression (c; r2 = 0.88, 0.67, and 0.51 for the AA, CA, and CC genotypes, respectively). The percentage of CD33 expressed as D2-CD33 was strongly associated with rs3865444 genotype (d, p = 1.2 × 10−13).
- Figure 3.
CD33 and TREM2 represent opposing forces in microglial activation. Sialic acid binding to CD33 results in activation of SHP1 phosphatase, which inhibits immune cell activation. The D2-CD33 isoform lacks the exon that encodes the apparent sialic acid binding domain and hence is proposed to be nonresponsive to sialic acid and inactive. In contrast, an unknown ligand binds TREM2 that signals through DAP12 to activate the tyrosine kinase Syk, resulting in microglial activation. Genetic evidence suggests that loss of CD33 function decreases AD risk, whereas loss of TREM2 function increases AD risk.
